The invention relates to the coating of a moving web-like material using high-pressure techniques and it concerns the nozzle used in such coating. The invention can be used especially in paper coating.
In paper coating, a coating composition is applied to the paper surface with a special view to enhancing the printing characteristics of paper. Conventionally, presses, knife applicators and film-transfer devices have been used for coating. These techniques are difficult to implement reliably, especially when an increase in the running speed or coating of very thin paper is required.
Spray coating has appeared as the most recent coating technique. It has the special advantage of not requiring any mechanical coating means, such as an abrasive knife or rotating rod, in contact with the web. High-pressure spray techniques have proved particularly promising. Here the coating composition alone, without any gaseous medium, is driven under high pressure through a nozzle with small orifices, the composition being diffused (atomized) into small droplets. The pressure may be e.g. in the range from 1 to 200 MPa and the nozzle orifice area e.g. in the range from 0.02 to 0.5 mm2. A typical maximum droplet size is approximately 100 μm. Such an apparatus comprises a nozzle array having one or more nozzle rows transverse to the path and consisting of a plurality of nozzles. The nozzles are disposed so as to cover the web as evenly as possible with the jets. Then jets formed by adjacent nozzles in a nozzle row overlap appropriately at their edges. The jet shape provided by the nozzle depends on the shape of the nozzle orifice. The usual aim is a fan-shaped jet, which is larger in the transverse direction than in the longitudinal direction of the web. Then the nozzle orifice is accordingly oval. To achieve regular coating, the fans are preferably disposed obliquely to the direction of travel of the web.
Spray coating of paper is described e.g. in the papers FI-B-108061 (corresponding to WO 9713036) and Nissinen V, OptiSpray, the New Low Impact Paper Coating Technology, OptiSpray Coating and Sizing Conference, Finland, Mar. 15, 2001.
Nozzles can be manufactured by making a piece of a suitable material, e.g. a highly wear-resistant material, the piece having a tapered duct ending in a closed tip, the desired nozzle orifice being subsequently machined in the tip. An oval orifice is provided if a transverse V-shaped groove is machined in the tip. The nozzle material may be e.g. a highly wear-resistant tungsten carbide composition (such as WC+Co).